The year 2018 is on course to be the fourth warmest on record, according to the World Meteorological Organization.
Other tell-tale signs of climate change were seen in 2018, including rising sea levels, increasingly acidic oceans, and widepread sea-ice and glacier melt.
Figures released by the WMO showed that the planet was almost 1C (1.8F) above pre-industrial levels for the first ten months of this year.
"Greenhouse gas concentrations are once again at record levels and if the current trend continues we may see temperature increases 3-5 degrees Celsius by the end of the century".
"It is worth repeating once again that we are the first generation to fully understand climate change and the last generation to be able to do something about it", he said.
The WMO Secretary-General's comments support the findings of another authoritative global body, The Intergovernmental Panel on Climate Change (IPCC).
It is to take effect in 2020 and calls for limiting global warming to less than 1.5C above pre-industrial levels.
More than 150 million vulnerable people worldwide were affected by potentially unsafe heatwaves a year ago, it said, while a total of 153 billion work hours were lost owing to unusually hot weather.
'It makes a difference to the speed of glacier melt and water supplies, and the future of low-lying islands and coastal communities.
The Paris Agreement, which was first signed in 2015, is an global agreement to control and limit climate change.
The meeting is to adopt the implementation guidelines of the Paris Agreement on Climate Change, which aims to hold the global average temperature increase to as close as possible to 1.5 degrees Celsius.
"Every extra bit matters", said Manaenkova.
A just-published United Kingdom assessment also warned that summer temperatures could be up to 5.4 degrees centigrade hotter, and summer rainfall could decrease by up to 47 per cent by 2070.
In the WMO's annual statement on the state of the climate, Secretary-General Petteri Taalas said that the rise would be "considerably higher" than that if all known fossil fuel resources were exploited.
Sea level around the United Kingdom will continue to rise to 2100 under all emission pathways, the report states, with sea levels in London potentially up 1.15m by 2100.
If El Niño develops, 2019 is likely to be warmer than 2018, scientists say. La Niña is a weather phenomenon associated with lower temperatures, while El Niño is associated with higher temperatures.
Jens Mattias Clausen, Greenpeace's head of delegation at COP24, said: "The evidence, if we needed any more, continues to stack up". Japan also experienced serious flooding, as did east Africa.